Optical coupling structure, system and method for preparing optical coupling structure

ABSTRACT

An optical coupling structure, an optical coupling system and a method for preparing the optical coupling structure are provided. The method includes: step S101: preparing a base substrate; step S102: forming a lithium niobate optical waveguide on the base substrate; step S103: forming a silicon dioxide core layer enclosing the lithium niobate optical waveguide on peripheral walls of the lithium niobate optical waveguide; step S104: forming a silicon dioxide cladding layer enclosing the silicon dioxide core layer on peripheral walls of the silicon dioxide core layer. The optical coupling structure alleviates a technical problem of low coupling efficiency between the lithium niobate optical waveguide and the single-mode optical fiber in the related art, and achieves a technical effect of improving the coupling efficiency between the lithium niobate optical waveguide and the single-mode optical fiber.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a National Stage Application of InternationalApplication No. PCT/CN2018/120911, filed on Dec. 13, 2018.

TECHNICAL FIELD

The present disclosure relates to a field of optical fiber communicationand integrated optics, in particular to an optical coupling structure, asystem and a method for preparing the optical coupling structure.

BACKGROUND

Conventional lithium niobate devices are prepared by processes such ashigh-temperature proton exchange, and have defects such as large devicevolume and poor process consistency. The emergence of the lithiumniobate etching process has made it possible to integrate multipleoptical functional devices on a lithium niobate wafer using processessuch as photolithography and etching.

In practical applications, a cross-sectional area of lithium niobateoptical waveguide is on an order of square microns, while across-sectional area of single-mode optical fiber is on an order ofhundreds of square microns. Because the cross-sectional area of lithiumniobate optical waveguide is different from that of single-mode opticalfiber on the order of magnitude, a mode field between the lithiumniobate optical waveguide and the single-mode optical fiber will notmatch. It leads to a problem of low coupling efficiency between thelithium niobate optical waveguide and the single-mode optical fiber.

SUMMARY (1) Technical Problems to be Solved

In view of the above problems, the present disclosure provides anoptical coupling structure, a system, and a method for preparing theoptical coupling structure, so as to at least partially solve the aboveexisting technical problems.

(2) Technical Solutions

According to an aspect of the present disclosure, there is provided anoptical coupling structure, including: a base substrate; a lithiumniobate optical waveguide formed on the base substrate; a silicondioxide core layer formed on peripheral walls of the lithium niobateoptical waveguide and enclosing the lithium niobate optical waveguide;and a silicon dioxide cladding layer formed on peripheral walls of thesilicon dioxide core layer and enclosing the silicon dioxide core layer.

In some embodiments, an end face of the silicon dioxide core layer isrectangular or trapezoidal, and an area of the end face of the silicondioxide core layer ranges from tens of square microns to hundreds ofsquare microns.

In some embodiments, the optical coupling structure further includes: anadhesive; where the adhesive is provided on end faces of the silicondioxide core layer and the silicon dioxide cladding layer in a lighttransmission direction, and a difference value between a refractiveindex of the adhesive and a refractive index of the silicon dioxide corelayer is less than 0.5.

According to another aspect of the present disclosure, there is alsoprovided an optical coupling system, including: a first single-modeoptical fiber, a second single-mode optical fiber, and the opticalcoupling structure; where a first end of the optical coupling structureis connected to the first single-mode optical fiber, a second end of theoptical coupling structure is connected to the second single-modeoptical fiber, and the first end and the second end are both located inthe light transmission direction.

In some embodiments, a difference value between a refractive index ofthe silicon dioxide core layer and a refractive index of the silicondioxide cladding layer is Δn, and a difference value between arefractive index of a core region of the single-mode optical fiber and acladding layer of the single-mode optical fiber is Δn′, and Δn−Δn′<0.5.

In some embodiments, the lithium niobate optical waveguide includes acuboid waveguide in middle, and a first quadrangular prism waveguide anda second quadrangular prism waveguide located at both ends of the cuboidwaveguide respectively, the first quadrangular prism waveguide is closeto the first end, an end face of the first quadrangular prism waveguideaway from the first single-mode optical fiber is connected to an endface of the cuboid waveguide, a distance between an end face of thefirst quadrangular prism waveguide close to the first single-modeoptical fiber and the first end is greater than 0, and a cross-sectionaldimension of the first quadrangular prism waveguide gradually decreasesin a direction away from the first single-mode optical fiber to close tothe first single-mode optical fiber; the second quadrangular prismwaveguide is close to the second end, an end face of the secondquadrangular prism waveguide away from the second single-mode opticalfiber is connected to an end face of the cuboid waveguide, a distancebetween the end face of the second quadrangular prism waveguide close tothe second single-mode optical fiber and the second end is greater than0, and a cross-sectional dimension of the second quadrangular prismwaveguide gradually decreases in a direction away from the secondsingle-mode optical fiber to close to the second single-mode opticalfiber.

According to another aspect of the present disclosure, there is provideda method for preparing an optical coupling structure, including: stepS101: preparing a base substrate; step S102: forming a lithium niobateoptical waveguide on the base substrate; step S103: forming a silicondioxide core layer enclosing the lithium niobate optical waveguide onperipheral walls of the lithium niobate optical waveguide; step S104:forming a silicon dioxide cladding layer enclosing the silicon dioxidecore layer on peripheral walls of the silicon dioxide core layer.

In some embodiments, the base substrate includes a quartz substratelayer, a silicon dioxide buried layer, and a lithium niobate thin filmlayer in order from bottom to top, and the forming a lithium niobateoptical waveguide on the base substrate includes: etching the lithiumniobate thin film layer by photoetching to form the lithium niobateoptical waveguide.

In some embodiments, the forming a silicon dioxide core layer enclosingthe lithium niobate optical waveguide on peripheral walls of the lithiumniobate optical waveguide includes: forming a silicon dioxide cappinglayer on the structure formed in step S102; planarizing the silicondioxide capping layer; removing a part of the silicon dioxide cappinglayer and a part of the silicon dioxide buried layer by photoetching,and etching to top of the quartz substrate layer. An etched silicondioxide capping layer and an etched silicon dioxide buried layer form asilicon dioxide core layer; a length of the silicon dioxide core layerin a light transmission direction is greater than a length of thelithium niobate optical waveguide in the light transmission direction.

In some embodiments, the forming a silicon dioxide cladding layerenclosing the silicon dioxide core layer on peripheral walls of thesilicon dioxide core layer includes: forming a silicon dioxide top layeron the structure formed in step S103, and the silicon dioxide top layerand the quartz substrate layer form the silicon dioxide cladding layer;the length of the silicon dioxide core layer in the light transmissiondirection is equal to a length of the silicon dioxide cladding layer inthe light transmission direction.

(3) Beneficial Effects

It may be seen from the above technical solutions that the opticalcoupling structure, a system and a method for preparing the opticalcoupling structure provided by the present disclosure have the followingbeneficial effects.

(1) The optical coupling structure includes a silicon dioxide core layerformed on peripheral walls of a lithium niobate optical waveguide andenclosing the lithium niobate optical waveguide; and a silicon dioxidecladding layer formed on peripheral walls of the silicon dioxide corelayer and enclosing the silicon dioxide core layer. Since an area of anend face of the silicon dioxide core layer is greater than across-sectional area of the lithium niobate optical waveguide, the areaof the end face of the silicon dioxide core layer is closer to across-sectional area of the single-mode optical fiber, which may reducean optical loss caused by a mode field mismatch. Therefore, a technicalproblem of low coupling efficiency between the lithium niobate opticalwaveguide and the single-mode optical fiber in the related art isalleviated, and a technical effect of improving the coupling efficiencybetween the lithium niobate optical waveguide and the single-modeoptical fiber is achieved.

(2) The end face of the silicon dioxide core layer is rectangular ortrapezoidal. The area of the end face of the silicon dioxide core layerranges from tens of square microns to hundreds of square microns. Thedifference value between the refractive index of the silicon dioxidecore layer and the refractive index of the silicon dioxide claddinglayer is Δn, and the difference value between the refractive index ofthe core region of the single-mode optical fiber and a refractive indexof the cladding layer of the single-mode optical fiber is Δn′, and n−Δn′<0.5, so as to cause the optical coupling structure to match the modefield of the single-mode optical fiber, and reduce a mode mismatch losscaused by a mode mutation. Therefore, the technical problem of lowcoupling efficiency between the lithium niobate optical waveguide andthe single-mode optical fiber existing in the related art may bealleviated, and a technical effect of improving the coupling efficiencybetween the lithium niobate optical waveguide and the single-modeoptical fiber is achieved.

(3) The optical coupling structure also includes: an adhesive, theadhesive is provided on the end faces of the silicon dioxide core layerand the silicon dioxide cladding layer in the light transmissiondirection, and the difference value between the refractive index of theadhesive and the refractive index of the silicon dioxide core layer isless than 0.5. Therefore, a Fresnel reflection loss caused by a suddenchange in refractive index is reduced during coupling of light from theoptical coupling structure to the single-mode optical fiber. Therefore,the technical problem of low coupling efficiency between the lithiumniobate optical waveguide and the single-mode optical fiber existing inthe related art may be alleviated, and a technical effect of improvingthe coupling efficiency between the lithium niobate optical waveguideand the single-mode optical fiber is achieved.

(4) The cross-sectional dimension of the first quadrangular prismwaveguide gradually decreases in a direction away from the firstsingle-mode optical fiber to close to the first single-mode opticalfiber. The cross-sectional dimension of the second quadrangular prismwaveguide gradually decreases in a direction away from the secondsingle-mode optical fiber to close to the second single-mode opticalfiber, so that an optical field may be slowly transited from the lithiumniobate optical waveguide to a silicon dioxide waveguide to reduce theoptical loss during the transition. Therefore, the technical problem oflow coupling efficiency between the lithium niobate optical waveguideand the single-mode optical fiber existing in the related art may bealleviated, and a technical effect of improving the coupling efficiencybetween the lithium niobate optical waveguide and the single-modeoptical fiber is achieved.

(5) In the present disclosure, the photoetching process is used toobtain the optical coupling structure, which avoids the problems of highprocess difficulty and poor device consistency caused by thehigh-temperature proton exchange process in the related art forpreparing lithium niobate devices. Therefore, the process difficulty ofpreparing the optical coupling structure is reduced, and the consistencyof the optical coupling structure is improved.

(6) In the present disclosure, no grating is used in the process ofpreparing the optical coupling structure, so the problem of sensitivityto the light polarization state caused by the use of gratings to preparethe optical coupling structure in the related art may be avoided, andtherefore, the optical coupling structure that is insensitive to thelight polarization state reduces the time that the workers use theoptical coupling structure and improves the use efficiency of theworkers.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to more clearly explain the specific embodiments of the presentdisclosure or the technical solutions in the related art, the followingwill briefly introduce the drawings that need to be used in the specificembodiments or the description of the related art. Obviously, thedrawings in the following description are some embodiments of thepresent disclosure. For those of ordinary skill in the art, otherdrawings may be obtained based on these drawings without creative work.

FIG. 1 is a flowchart of a method for preparing an optical couplingstructure according to an embodiment of the present disclosure.

FIG. 2 is a front view of a base substrate before a lithium niobateoptical waveguide is formed according to an embodiment of the presentdisclosure.

FIG. 3 is a front view of a base substrate after a lithium niobateoptical waveguide is formed according to an embodiment of the presentdisclosure.

FIG. 4 is a top view of a base substrate after a lithium niobate opticalwaveguide is formed according to an embodiment of the presentdisclosure.

FIG. 5 is a front view of a silicon dioxide waveguide before a silicondioxide core layer is formed according to an embodiment of the presentdisclosure.

FIG. 6 is a front view of a silicon dioxide waveguide after a silicondioxide core layer is formed according to an embodiment of the presentdisclosure.

FIG. 7 is a top view of a silicon dioxide waveguide after a silicondioxide core layer is formed according to an embodiment of the presentdisclosure.

FIG. 8 is a front view of an optical coupling structure according to anembodiment of the present disclosure.

FIG. 9 is a first top view of an optical coupling system according to anembodiment of the present disclosure.

FIG. 10 is a second top view of an optical coupling system according toan embodiment of the present disclosure.

FIG. 11 is a third top view of an optical coupling system according toan embodiment of the present disclosure.

FIG. 12 is a fourth top view of an optical coupling system according toan embodiment of the present disclosure.

DETAILED DESCRIPTION OF EMBODIMENTS

In order to make the objectives, technical solutions, and advantages ofthe present disclosure clearer, the following further describes thepresent disclosure in detail in conjunction with specific embodimentsand with reference to the drawings.

An optical coupling structure, a system and a method for preparing theoptical coupling structure provided by the embodiments of the presentdisclosure may alleviate a technical problem of low coupling efficiencybetween a lithium niobate optical waveguide and a single-mode opticalfiber in the related art, and achieve a technical effect of improvingthe coupling efficiency between the lithium niobate optical waveguideand the single-mode optical fiber.

In order to facilitate the understanding of the embodiments of thepresent disclosure, a method for preparing an optical coupling structureaccording to an embodiment of the present disclosure is first introducedin detail. As shown in FIG. 1, the method for preparing an opticalcoupling structure may include the following steps.

Step S101: A base substrate is prepared.

Where, as shown in FIG. 2, the base substrate may include a quartzsubstrate layer 11, a silicon dioxide buried layer 12, and a lithiumniobate thin film layer 13 in order from bottom to top. A materialcomponent of the quartz substrate layer 11 may be pure silicon dioxide,or may be doped silicon dioxide. The quartz substrate layer 11 may playa role in supporting the entire optical coupling structure. The silicondioxide buried layer 12 may be a deposited silicon dioxide adjusted by adeposition (sedimentation) process. A material component of the silicondioxide buried layer 12 may be pure silicon dioxide, or may be dopedsilicon dioxide. The lithium niobate thin film layer 13 may be bonded tothe silicon dioxide buried layer 12 by a special process. A materialcomponent of the lithium niobate thin film layer 13 may be pure lithiumniobate, or may be a doped lithium niobate material.

Further, a refractive index range of the quartz substrate layer 11 forlight with a wavelength of 1550 nm is approximately 1.4 to 1.6. Arefractive index range of the silicon dioxide buried layer 12 for lightwith a wavelength of 1550 nm is approximately 1.4 to 1.6.

It should be noted that the refractive index of the silicon dioxideburied layer 12 is higher than the refractive index of the quartzsubstrate layer 11.

Step S102: A lithium niobate optical waveguide is formed on the basesubstrate.

Where, step S102 may include: as shown in FIG. 3, etching the lithiumniobate thin film layer 13 by photoetching to form a lithium niobateoptical waveguide 14. As shown in FIG. 4, the lithium niobate opticalwaveguide 14 may include a cuboid waveguide 15 in middle, and a firstquadrangular prism waveguide 161 and a second quadrangular prismwaveguide 162 located at both ends of the cuboid waveguide 15respectively.

Step S103: A silicon dioxide core layer enclosing the lithium niobateoptical waveguide is formed on peripheral walls of the lithium niobateoptical waveguide.

Preferably, a shape of an end face of the silicon dioxide core layer isrectangular or trapezoidal, and an area of the end face of the silicondioxide core layer ranges from tens of square microns to hundreds ofsquare microns.

Where, forming a silicon dioxide core layer enclosing the lithiumniobate optical waveguide on peripheral walls of the lithium niobateoptical waveguide may include the following steps.

As shown in FIG. 5, a silicon dioxide capping layer 17 is formed on thestructure formed in step S102.

Exemplarily, the silicon dioxide capping layer 17 may be pure silicondioxide or doped silicon dioxide formed by a deposition (sedimentation)process. The silicon dioxide capping layer 17 covers the lithium niobateoptical waveguide 14. A difference value between a refractive index ofthe silicon dioxide capping layer 17 and the refractive index of thequartz substrate layer 11 may be set within 10%. Specifically, arefractive index range of the silicon dioxide capping layer 17 for lightwith a wavelength of 1550 nm is approximately 1.4 to 1.6.

Specifically, a plasma-enhanced chemical vapor deposition PECVD processmay be used to form the silicon dioxide capping layer 17, and silicondioxide is generated by a reaction of silane and nitric oxide at 350degrees Celsius. The reaction equation is as follows: SiH₄ (gaseousstate)+2N₂O (gaseous state)-SiO₂ (solid state)+2N₂ (gaseous state)+2H₂(gaseous state).

The silicon dioxide capping layer 17 is planarized.

Exemplarily, a chemical mechanical polishing may be used to planarizethe silicon dioxide capping layer.

As shown in FIG. 6, on the silicon dioxide capping layer that hasundergone the planarization treatment, a part of the silicon dioxidecapping layer and a part of the silicon dioxide buried layer are removedby photoetching. Etching is performed to top of the quartz substratelayer 11. An etched silicon dioxide capping layer 19 and an etchedsilicon dioxide buried layer 18 form a silicon dioxide core layer. Asshown in FIG. 7, the silicon dioxide core layer completely encloses thelithium niobate optical waveguide 14.

Specifically, after coating a photoresist on the silicon dioxide cappinglayer, a mask is used for exposure and development to transfer a maskpattern to the photoresist. Then, the pattern on the photoresist istransferred to the silicon dioxide capping layer by etching. A mixed gasof CF₄ and H₂ is selected as a gas for etching silicon dioxide. Acontent of H₂ in the mixed gas is 50% of the volume of the mixed gas. Aselection ratio of the mixed gas of CF₄/H₂ with this component tosilicon dioxide and silicon exceeds 40:1, and the etching selectivity isbetter. In plasma environment, CF₄ may produce fluorine atoms, whichreact with silicon dioxide to etch silicon dioxide. The reactionequation is as follows:

CF₄ +e−-CF₃+F+e−

4F(radical)+SiO₂ (solid state)-SiF₄ (gaseous state)+O₂ (gaseous state)

Where, the function of H₂ is to reduce a reaction rate of CF₄ andsilicon, and to increase a selective etching ratio of CF₄ to silicondioxide and silicon. This etching process is performed to the quartzsubstrate layer.

It should be noted that the refractive index of the silicon dioxideburied layer may be equal to the refractive index of the silicon dioxidecapping layer, and the refractive index of the silicon dioxide buriedlayer may also be close to that of the silicon dioxide capping layer.The refractive index of the silicon dioxide capping layer is higher thanthe refractive index of the quartz substrate layer. To obtain arefractive index of the silicon dioxide capping layer that meetsconditions, it may be achieved by adjusting a ratio of SiH₄ and 2N₂O ina chemical method SiH₄ (gaseous state)+2N₂O (gaseous state)-SiO₂ (solidstate)+2N₂ (gaseous state)+2H₂ (gaseous state), and it may also beachieved by doping. If the refractive index of the silicon dioxideburied layer is equal to the refractive index of the silicon dioxidecapping layer, the refractive index of the silicon dioxide buried layermay be determined as the refractive index of the silicon dioxide corelayer. If the refractive index of the silicon dioxide buried layer isclose to that of the silicon dioxide capping layer, an average value ofthe refractive index of the silicon dioxide buried layer and therefractive index of the silicon dioxide capping layer may be determinedas the refractive index of the silicon dioxide core layer.

Where a length of the silicon dioxide core layer in a light transmissiondirection is greater than a length of the lithium niobate opticalwaveguide in the light transmission direction. In other words, thesilicon dioxide core layer not only completely encloses the lithiumniobate optical waveguide, but also an end part of the silicon dioxidecore layer does not enclose the lithium niobate optical waveguide, andthe end part of the silicon dioxide core layer is completely silicondioxide waveguide.

Step S104: A silicon dioxide cladding layer enclosing the silicondioxide core layer is formed on peripheral walls of the silicon dioxidecore layer.

Where, as shown in FIG. 8, forming a silicon dioxide cladding layerenclosing the silicon dioxide core layer on peripheral walls of thesilicon dioxide core layer includes: forming a silicon dioxide top layer20 on the structure formed in step S103. The silicon dioxide top layer20 and the quartz substrate layer 11 form the silicon dioxide claddinglayer. The length of the silicon dioxide core layer in the lighttransmission direction is equal to a length of the silicon dioxidecladding layer in the light transmission direction.

Exemplarily, the silicon dioxide top layer 20 may be pure silicondioxide or doped silicon dioxide formed by a deposition (sedimentation)process. Specifically, the silicon dioxide top layer 20 may be adeposited silicon dioxide adjusted by a deposition process.Specifically, a refractive index range of the silicon dioxide top layer20 for light with a wavelength of 1550 nm is approximately 1.4 to 1.6.

It should be noted that a refractive index of the silicon dioxide toplayer may be equal to the refractive index of the quartz substratelayer, and the refractive index of the silicon dioxide top layer mayalso be close to the refractive index of the quartz substrate layer. Therefractive index of the silicon dioxide top layer is lower than that ofthe silicon dioxide buried layer. The refractive index of the silicondioxide top layer is lower than that of the silicon dioxide cappinglayer. The refractive index of the silicon dioxide top layer and therefractive index of the quartz substrate layer are both lower than therefractive index of the silicon dioxide core layer. If the refractiveindex of the silicon dioxide top layer is equal to the refractive indexof the quartz substrate layer, the refractive index of the silicondioxide top layer may be determined as the refractive index of thesilicon dioxide capping layer. If the refractive index of the silicondioxide top layer is close to the refractive index of the quartzsubstrate layer, an average value of the refractive index of the silicondioxide top layer and the refractive index of the quartz substrate layermay be determined as the refractive index of the silicon dioxidecladding layer.

This embodiment has following beneficial effects:

(1) a silicon dioxide core layer enclosing the lithium niobate opticalwaveguide is formed on peripheral walls of the lithium niobate opticalwaveguide. A silicon dioxide cladding layer enclosing the silicondioxide core layer is formed on peripheral walls of the silicon dioxidecore layer. Since the area of the end face of the silicon dioxide corelayer is greater than the cross-sectional area of the lithium niobateoptical waveguide, the area of the end face of the silicon dioxide corelayer is closer to a cross-sectional area of a single-mode opticalfiber, an optical loss caused by mode field mismatch may be reduced. Itmay alleviate a technical problem of low coupling efficiency between thelithium niobate optical waveguide and the single-mode optical fiber inrelated art, so as to achieve a technical effect of improving thecoupling efficiency between the lithium niobate optical waveguide andthe single-mode optical fiber;

(2) the optical coupling structure is obtained through aphotolithography process and an etching process, thereby avoiding highprocess difficulty and poor device consistency caused by ahigh-temperature proton exchange process which is used to preparelithium niobate devices in the related art, therefore the processdifficulty of preparing the optical coupling structure may be reducedand the consistency of the optical coupling structure is improved;

(3) in the process of preparing the optical coupling structure, nograting is used, thereby avoiding a problem of sensitivity to a lightpolarization state caused by the use of gratings to prepare the opticalcoupling structure in the related art. Therefore, an optical couplingstructure that is insensitive to the light polarization state may beobtained, which reduces time for workers to use the optical couplingstructure, thereby improving the use efficiency of the workers.

In yet another embodiment of the present disclosure, an optical couplingstructure described in an embodiment of the present disclosure isintroduced in detail. The optical coupling structure includes a basesubstrate.

Where, as shown in FIG. 2, the base substrate may include a quartzsubstrate layer 11, a silicon dioxide buried layer 12, and a lithiumniobate thin film layer 13 in order from bottom to top.

A lithium niobate optical waveguide is formed on the base substrate.

Where, as shown in FIGS. 3 and 4, the lithium niobate optical waveguide14 may include a cuboid waveguide 15 in middle, and a first quadrangularprism waveguide 161 and a second quadrangular prism waveguide 162located at both ends of the cuboid waveguide 15 respectively.Exemplarily, a width and a height of the cuboid waveguide 15 may rangefrom 1 to 20 microns.

A silicon dioxide core layer is formed on peripheral walls of thelithium niobate optical waveguide and encloses the lithium niobateoptical waveguide.

Preferably, a shape of an end face of the silicon dioxide core layer isrectangular or trapezoidal, and an area of the end face of the silicondioxide core layer ranges from tens of square microns to hundreds ofsquare microns.

A silicon dioxide cladding layer is formed on peripheral walls of thesilicon dioxide core layer and encloses the silicon dioxide core layer.

In an embodiment of the present disclosure, the optical couplingstructure may further include an adhesive.

The adhesive is provided on an end face of the silicon dioxide corelayer and an end face of the silicon dioxide cladding layer in a lighttransmission direction, and a difference value between a refractiveindex of the adhesive and the refractive index of the silicon dioxidecore layer is less than 0.5.

Where the adhesive may be used to fix the single-mode optical fiber andthe end face of the silicon dioxide core layer and the end face of thesilicon dioxide cladding layer in the light transmission direction.

Further, after aligning the end face of the silicon dioxide core layerand the end face of the silicon dioxide cladding layer in the lighttransmission direction with the single-mode optical fiber, fixing may beperformed by using an adhesive. In an alignment process, a ∇-shapedgroove may be etched on the quartz substrate layer first, and then thesingle-mode optical fiber may be placed in the ∇-shaped groove, andfinally fixing may be perform by using a cover plate and an adhesive.The optical coupling structure in an embodiment of the presentdisclosure has a large optical bandwidth.

For example, FIG. 10, FIG. 11, and FIG. 12 schematically show opticalcoupling structures with different silicon dioxide waveguide endstructures in a case that the silicon dioxide core layer completelyencloses the lithium niobate optical waveguide. FIG. 10 is a schematicdiagram of an optical coupling structure having an end structure with aconstant width. FIG. 11 is a schematic diagram of an optical couplingstructure having an end structure whose width gradually narrows in adirection away from the single-mode optical fiber to close to thesingle-mode optical fiber. FIG. 12 is a schematic diagram of an opticalcoupling structure having an end structure whose width gradually widensin a direction away from the single-mode optical fiber to close to thesingle-mode optical fiber. The optical coupling structure in actualapplication should include but not limited to these three situations.

Where, as shown in FIG. 10, an end face of the silicon dioxide corelayer and an end face of the silicon dioxide cladding layer in the lighttransmission direction is connected to a single-mode optical fiber 213by an adhesive 223, and another end face of the silicon dioxide corelayer and another end face of the silicon dioxide cladding layer isconnected to a single-mode optical fiber 214 by an adhesive 224. Asshown in FIG. 11, an end face of the silicon dioxide core layer and anend face of the silicon dioxide cladding layer in the light transmissiondirection is connected to a single-mode optical fiber 215 by an adhesive225, and another end face of the silicon dioxide core layer and anotherend face of the silicon dioxide cladding layer is connected to asingle-mode optical fiber 216 by an adhesive 226. As shown in FIG. 12,an end face of the silicon dioxide core layer and an end face of thesilicon dioxide cladding layer in the light transmission direction isconnected to a single-mode optical fiber 217 by an adhesive 227, andanother end face of the silicon dioxide core layer and another end faceof the silicon dioxide cladding layer is connected to a single-modeoptical fiber 218 by an adhesive 228.

This embodiment has following beneficial effects: since the opticalcoupling structure further includes an adhesive, the adhesive isprovided on the end faces of the silicon dioxide core layer and thesilicon dioxide cladding layer in the light transmission direction, andthe difference value between the refractive index of the adhesive andthe refractive index of the silicon dioxide core layer is less than 0.5,therefore a Fresnel reflection loss caused by a sudden change inrefractive index is reduced during coupling of light from the opticalcoupling structure to the single-mode optical fiber. Furthermore, atechnical problem of low coupling efficiency between the lithium niobateoptical waveguide and the single-mode optical fiber existing in therelated art may be alleviated, and it is beneficial to further improvethe coupling efficiency between the lithium niobate optical waveguideand the single-mode optical fiber.

In another embodiment of the present disclosure, an optical couplingsystem in embodiments of the present disclosure is introduced in detail.The optical coupling system may include: a first single-mode opticalfiber, a second single-mode optical fiber, and the optical couplingstructure as described in the above embodiments.

A first end of the optical coupling structure is connected to the firstsingle-mode optical fiber, a second end of the optical couplingstructure is connected to the second single-mode optical fiber, and thefirst end of the optical coupling structure and the second end of theoptical coupling structure are both located in the light transmissiondirection.

As shown in FIG. 9, a first end 231 of the optical coupling structuremay be connected to a first single-mode optical fiber 211 by an adhesive221, and a second end 232 of the optical coupling structure may beconnected to a second single-mode optical fiber 212 by an adhesive 222.

Preferably, a difference value between the refractive index of thesilicon dioxide core layer and the refractive index of the silicondioxide cladding layer is Δn, and a difference value between arefractive index of a core region of the single-mode optical fiber and arefractive index of a cladding layer of the single-mode optical fiber isΔn′, and n−Δn′<0.5.

Where, as shown in FIG. 9, the lithium niobate optical waveguide mayinclude a cuboid waveguide 15 in middle, and a first quadrangular prismwaveguide 161 and a second quadrangular prism waveguide 162 located atboth ends of the cuboid waveguide 15 respectively.

Preferably, the first quadrangular prism waveguide 161 is close to thefirst end 231, and an end face of the first quadrangular prism waveguide161 away from the first single-mode optical fiber 211 is connected to anend face of the cuboid waveguide 15. A distance between the end face ofthe first quadrangular prism waveguide 161 that is close to the firstsingle-mode optical fiber 211 and the first end 231 is greater than 0. Across-sectional dimension of the first quadrangular prism waveguide 161gradually decreases in a direction away from the first single-modeoptical fiber 211 to close to the first single-mode optical fiber 211.The second quadrangular prism waveguide 162 is close to the second end232, and an end face of the second quadrangular prism waveguide 162 awayfrom the second single-mode optical fiber 212 is connected to an endface of the cuboid waveguide 15. A distance between the end face of thequadrangular prism waveguide 162 that is close to the second single-modeoptical fiber 212 and the second end 232 is greater than 0. Across-sectional dimension of the second quadrangular prism waveguide 162gradually decreases in a direction away from the second single-modeoptical fiber 212 to close to the second single-mode optical fiber 212.

A perpendicular distance between an end face of the first quadrangularprism waveguide 161 close to the first single-mode optical fiber 211 andan end face of the first quadrangular prism waveguide 161 away from thefirst single-mode optical fiber 211 is greater than 0.5 microns. Aperpendicular distance between an end face of the second quadrangularprism waveguide 162 close to the second single-mode optical fiber 212and an end face of the second quadrangular prism waveguide 162 away fromthe second single-mode optical fiber 212 is greater than 0.5 microns.

Preferably, to ensure a single-mode transmission, a preferred valuerange of a width and a height of the cuboid waveguide 15 may be 1 to 1.5microns. The perpendicular distance between the end face of the firstquadrangular prism waveguide 161 close to the first single-mode opticalfiber 211 and the end face of the first quadrangular prism waveguide 161away from the first single-mode optical fiber 211 may be 200 microns.The perpendicular distance between the end face of the secondquadrangular prism waveguide 162 close to the second single-mode opticalfiber 212 and the end face of the second quadrangular prism waveguide162 away from the second single-mode optical fiber 212 may be 200microns, so that an optical field may be slowly transited from thelithium niobate optical waveguide to a silicon dioxide waveguide to beformed later.

A shape of the end face of the silicon dioxide core layer is rectangularor trapezoidal. An area of the end face of the silicon dioxide corelayer ranges from tens of square microns to hundreds of square microns.The difference value between the refractive index of the silicon dioxidecore layer and the refractive index of the silicon dioxide claddinglayer and the difference value between the refractive index of the coreregion the single-mode optical fiber and the cladding layer of thesingle-mode optical fiber is less than 0.5, so as to cause the opticalcoupling structure to match the mode field of the single-mode opticalfiber, and reduce a mode mismatch loss caused by a mode mutation.Therefore, the technical problem of low coupling efficiency between thelithium niobate optical waveguide and the single-mode optical fiberexisting in the related art may be alleviated, which is beneficial tofurther improve the coupling efficiency between the lithium niobateoptical waveguide and the single-mode optical fiber.

The cross-sectional dimension of the first quadrangular prism waveguidegradually decreases in a direction away from the first single-modeoptical fiber to close to the first single-mode optical fiber. Thecross-sectional dimension of the second quadrangular prism waveguidegradually decreases in a direction away from the second single-modeoptical fiber to close to the second single-mode optical fiber. Itallows an optical field to be slowly transited from the lithium niobateoptical waveguide to the silicon dioxide waveguide, and reduces anoptical loss during the transition. Therefore, the technical problem oflow coupling efficiency between the lithium niobate optical waveguideand the single-mode optical fiber existing in the related art may bealleviated, which is beneficial to further improve the couplingefficiency between the lithium niobate optical waveguide and thesingle-mode optical fiber.

The specific embodiments described above further describe the purpose,technical solutions and beneficial effects of the present disclosure infurther detail. It should be understood that the above descriptions areonly specific embodiments of the present disclosure and are not intendedto limit the present disclosure. Any modification, equivalentreplacement, improvement, etc. made within the spirit and principle ofthe present disclosure shall be included in the protection scope of thepresent disclosure.

So far, the embodiments of the present disclosure have been described indetail with reference to the drawings. Based on the above description,those skilled in the art should have a clear understanding of thepresent disclosure.

It should be noted that, in the drawings or the main body of thespecification, the embodiments that are not shown or described are allforms known to those of ordinary skill in the art, and are not describedin detail. In addition, the above definitions of various elements andmethods are not limited to the various specific structures, shapes ormethods mentioned in the embodiments, and those of ordinary skill in theart may simply modify or replace them.

Of course, according to actual needs, the method of the presentdisclosure also includes other steps, which are not described herebecause they have nothing to do with the innovations of the presentdisclosure.

In addition, the above definitions of various elements and methods arenot limited to the various specific structures, shapes or methodsmentioned in the embodiments, and those of ordinary skill in the art maysimply modify or replace them.

It should be noted that the directional terms mentioned in theembodiments, such as “upper”, “lower”, “front”, “rear”, “left”, “right”,etc., only refer to the directions of the drawings, and are not used tolimit the protection scope of this disclosure. Throughout the drawings,the same elements are represented by the same or similar referencenumerals. When it may cause confusion in the understanding of thepresent disclosure, conventional structures or configurations will beomitted. The shape and size of each component in the figure do notreflect the actual size and proportion, but merely illustrate thecontent of the embodiment of the present disclosure. In addition, in theclaims, any reference signs located between parentheses should not beconstructed as limitations on the claims.

Unless clearly indicated, the numerical parameters in this specificationand the appended claims are approximate values and may be changedaccording to the required characteristics obtained through the contentof the present disclosure. Specifically, all numbers used in thespecification and claims to indicate the content of the composition,reaction conditions, etc., should be understood as being modified by theterm “approximate” in all cases. In general, the meaning of itsexpression refers to a change of ±10% in some embodiments, a change of±5% in some embodiments, a change of ±1% in some embodiments, and achange of ±0.5% in some embodiments.

The word “comprising” or “including” does not exclude the presence ofelements or steps not listed in the claims. The word “a” or “an”preceding an element does not exclude the presence of multiple suchelements.

The ordinal numbers used in the specification and claims, such as termssuch as “first”, “second”, and “third”, are used to modify thecorresponding element. It does not mean that the element has any ordinalnumber, nor does it represent an order of a certain element and anotherelement, or the order of the manufacturing method. The use of theseordinal numbers is only used to clearly distinguish an element with acertain name from another element with the same name.

Similarly, it should be understood that in order to simplify the presentdisclosure and help understand one or more of the various disclosedaspects, in the above description of the exemplary embodiments of thepresent disclosure, the various features of the present disclosure aresometimes grouped together into a single embodiment, figure, or itsdescription. However, the disclosed method should not be interpreted asreflecting the intention that the claimed disclosure requires morefeatures than those explicitly recorded in each claim. More precisely,as reflected in the following claims, the disclosure aspect lies in lessthan all the features of a single embodiment previously disclosed.Therefore, the claims following the specific embodiment are thusexplicitly incorporated into the specific embodiment, where each claimitself serves as a separate embodiment of the present disclosure.

The specific embodiments described above further describe the purpose,technical solutions, and beneficial effects of the present disclosure infurther detail. It should be understood that the above descriptions areonly specific embodiments of the present disclosure and are not intendedto limit the present disclosure. Any modification, equivalentreplacement, improvement, etc., made within the spirit and principle ofthe present disclosure shall be included in the protection scope of thepresent disclosure.

What is claimed is:
 1. An optical coupling structure, comprising: a basesubstrate; a lithium niobate optical waveguide formed on the basesubstrate; a silicon dioxide core layer formed on peripheral walls ofthe lithium niobate optical waveguide and enclosing the lithium niobateoptical waveguide; and a silicon dioxide cladding layer formed onperipheral walls of the silicon dioxide core layer and enclosing thesilicon dioxide core layer.
 2. The optical coupling structure accordingto claim 1, wherein an end face of the silicon dioxide core layer isrectangular or trapezoidal, and an area of the end face of the silicondioxide core layer ranges from tens of square microns to hundreds ofsquare microns.
 3. The optical coupling structure according to claim 2,further comprises an adhesive, wherein the adhesive is provided on endfaces of the silicon dioxide core layer and the silicon dioxide claddinglayer in a light transmission direction, and a difference value betweena refractive index of the adhesive and a refractive index of the silicondioxide core layer is less than 0.5.
 4. An optical coupling system,comprising: a first single-mode optical fiber, a second single-modeoptical fiber, and the optical coupling structure according to any oneof claims 1 to 3; wherein a first end of the optical coupling structureis connected to the first single-mode optical fiber, a second end of theoptical coupling structure is connected to the second single-modeoptical fiber, and the first end and the second end are both located inthe light transmission direction.
 5. The optical coupling systemaccording to claim 4, wherein a difference value between a refractiveindex of the silicon dioxide core layer and a refractive index of thesilicon dioxide cladding layer is Δn, and a difference value between arefractive index of a core region of the single-mode optical fiber and acladding layer of the single-mode optical fiber is Δn′, and Δn−Δn′<0.5.6. The optical coupling system according to claim 5, wherein the lithiumniobate optical waveguide comprises a cuboid waveguide in middle, and afirst quadrangular prism waveguide and a second quadrangular prismwaveguide located at both ends of the cuboid waveguide respectively;wherein the first quadrangular prism waveguide is close to the firstend, an end face of the first quadrangular prism waveguide away from thefirst single-mode optical fiber is connected to an end face of thecuboid waveguide, a distance between an end face of the firstquadrangular prism waveguide close to the first single-mode opticalfiber and the first end is greater than 0, and a cross-sectionaldimension of the first quadrangular prism waveguide gradually decreasesin a direction away from the first single-mode optical fiber to close tothe first single-mode optical fiber; and wherein the second quadrangularprism waveguide is close to the second end, an end face of the secondquadrangular prism waveguide away from the second single-mode opticalfiber is connected to an end face of the cuboid waveguide, a distancebetween the end face of the second quadrangular prism waveguide close tothe second single-mode optical fiber and the second end is greater than0, and a cross-sectional dimension of the second quadrangular prismwaveguide gradually decreases in a direction away from the secondsingle-mode optical fiber to close to the second single-mode opticalfiber.
 7. A method for preparing an optical coupling structure,comprising: step S101: preparing a base substrate; step S102: forming alithium niobate optical waveguide on the base substrate; step S103:forming a silicon dioxide core layer enclosing the lithium niobateoptical waveguide on peripheral walls of the lithium niobate opticalwaveguide; step S104: forming a silicon dioxide cladding layer enclosingthe silicon dioxide core layer on peripheral walls of the silicondioxide core layer.
 8. The method for preparing an optical couplingstructure according to claim 7, wherein the base substrate comprises aquartz substrate layer, a silicon dioxide buried layer, and a lithiumniobate thin film layer in order from bottom to top; wherein the forminga lithium niobate optical waveguide on the base substrate comprises:etching the lithium niobate thin film layer by photoetching to form thelithium niobate optical waveguide.
 9. The method for preparing anoptical coupling structure according to claim 8, wherein the forming asilicon dioxide core layer enclosing the lithium niobate opticalwaveguide on peripheral walls of the lithium niobate optical waveguidecomprises: forming a silicon dioxide capping layer on a structure formedin step S102; planarizing the silicon dioxide capping layer; removing apart of the silicon dioxide capping layer and a part of the silicondioxide buried layer by photoetching, and etching is performed to thetop of the quartz substrate layer; wherein an etched silicon dioxidecapping layer and an etched silicon dioxide buried layer form thesilicon dioxide core layer; and a length of the silicon dioxide corelayer in a light transmission direction is greater than a length of thelithium niobate optical waveguide in the light transmission direction.10. The method for preparing an optical coupling structure according toclaim 9, wherein the forming a silicon dioxide cladding layer enclosingthe silicon dioxide core layer on peripheral walls of the silicondioxide core layer comprises: forming a silicon dioxide top layer on astructure formed in step S103, wherein the silicon dioxide top layer andthe quartz substrate layer form the silicon dioxide cladding layer; andthe length of the silicon dioxide core layer in the light transmissiondirection is equal to a length of the silicon dioxide cladding layer inthe light transmission direction.